Grind 1 tbsp. finely chopped white onion, 1 firmly packed tbsp. chopped fresh cilantro, 2 tsp. finely chopped jalapeño, and 1 tsp. salt together in a mortar and pestal until all the ingredients are well ground. (Alternatively, use a fork to mash the ingredients to a paste in a wide bowl.)

Cut 3 medium-ripe hass avocados (about 8 ounces each) in half. Twist the halves to separate them and remove the pit with the tip of the knife. Place an avocado half, cut side up, in your palm and make 3 or 4 evenly spaced lengthwise cuts through its flesh down to the skin, without cutting through the skin. Make 4 crosswise cuts in the same fashion.

Scoop the diced avocado flesh into the mortar and pestal. Repeat with remaining avocado halves. Gently fold the avocado into the chile–onion paste, keeping the avocado pieces fairly intact.